Recording elastic-wave phase holographic data

ABSTRACT

In elastic-wave holography where transducers are used to convert the detected elastic waves into corresponding electrical signals, the phase pattern of many transducers spread over the hologram area is recorded by sampling all of the electrical signals during one cycle of the coherent elastic waves to get a relative time delay for the zero-axis crossings of each signal. Pulses at zeroaxis crossing times, produced by amplifying, clipping, and differentiating the signals, are then shaped or stretched to form short square waves of time duration M, or phase duration K electrical degrees, where K equals 360*/L, L being the integral number of phase intervals into which the coherent-wave cycle is subdivided. Each value of K also corresponds to one step of the gray scale used in recording the final hologram. To record the hologram directly or in a form suitable for later playback, all transducer outputs during one coherent-wave cycle are scanned L times, once for each successive K* interval, to detect the square-wave pulses. In each scan, the occurrence times of pulses correspond to those transducer positions in the hologram area having relative phases within the given K* interval. These are accordingly the positions for recording a particular gray-scale density on the final hologram. Succeeding scans detect the positions of different phase and different corresponding density on the hologram, until all phase intervals and hologram density points have been covered.

United States Patent Silverman [151 3,678,452 [451 July 18,1972

[54] RECORDING ELASTIG WAVEPHASE HOLOGRAPHIC DATA' 221 Filed: April 30,1970' 211 Appl. No.: 33,411

Rama u.s. Application pm [63] Continuation-impart of Ser. Nos. 824,925,May 15,

1969, and Set. N0. 858,635, Sept. 17, 1969.

[52] US. Cl. ..340/3 R, 73/675 H, 340/5 H,

v 340/155 R 51 m. G0ls 9/66 58 FleldofSeareh ..340/3 R, 3 F, 5 H, 15.5DP; 73/67.5 11; 181/05 NP [56] References Cited UNITED STATES PATENTS2,364,209 12/1944 Green ..1s1/0.5 3,323,105 5/1967 Charske......340/1s.5 DP

[ ABSTRACT ln elastic-wave holography where transducers are used toconvert the detected elastic waves into corresponding electricalsignals, the phase pattern of many transducers spread over the hologramarea is recorded by sampling all of the electrical signals during onecycle of the coherent elastic waves to get a relative time delay for thezero-axis crossings of each signal.

Pulses at zero-axis crossing times, produced by amplifying, clipping,and differentiating the signals, are then-shaped or stretched to fonnshort square waves of time duration M, or phase duration K electricaldegrees, where K equals 360IL, L being the integral number of phaseintervals into which the coherent-wave cycle is subdivided. Each valueof K also corresponds to one step of the gray scale used in recordingthe final hologram.

To record the hologram directly or in a form suitable for laterplayback, all transducer outputs during one coherent-wave cycle arescanned L times, once for each successive K interval, to detect thesquare-wave pulses. in each scan, the occurrence times of pulsescorrespond to those transducer positions in the hologram area havingrelative phases within the given K interval. These areaccordingly thepositions for recording a particular gray-scale density onthe finalhologram. Succeed- H ans dete t the of difiercnt and difi'e cn!3,506,952 4/1970 Gabor et al. ..340/3 R corresponding density on thehologram, il n phasc i vals and hologram density points have beencovered. Primary Examiner-Richard A. Farley Attorney-Paul F. l-lawleyand Newell Pottorf 1 1 Claim, 6 Drawing Figures PULSE LIMITERDIFFERENTIATER SHAPER TRIGGER I 25 37 38 I 1 SAMPLE FREQUENCY CLOCK jAND MULTI PLIER PULSES 39 I HOLD l W l MULTIPLEX :1 [8 SCAN AND 3-; '1

RECORDER 1 1 280 CONTROL CONTROL 4 i i: OSCILLATOR l L RECORDER 260 I9 3POWER AMPLIFIER 2 I ARRAY we 15 13 17\ l Patented July 18, 1972 3Sheets-Sheet z Patented Jul 18, 1972 3 Sheets-Sheet 5 5 3 E L L O P H ML; A D 3 IIY W ,.::i 4 r II II lllL R 3 C C C 3 m F RECORDER 36 FIG. 5

BEAM

OFF

SAWTOOTH G E NER ATOR FIG 6 INVENTOR. DANIEL SILVERMAN ATTORNEYRECORDING ELASTIC-WAVE PHASE HOLOGRAPIIIC DATA CROSS REFERENCE TORELATED APPLICATIONS This invention is a continuation-in-part of myco-pending applications Ser. No. 824,925, entitled Elastic-WaveHolography Using Reflections," filed May 15, 1969, and Ser. No. 858,635,entitled Elastic-Wave Holography of Elongated Objects, filed Sept. l7,I969. It is also related to my issued U. S. Pat. Nos. 3,400,363,3,450,225, and 3,461,420.

BACKGROUND OF THE INVENTION l. Field of the Invention This inventionrelates to elastic-wave holography and is directed particularly toelastic-wave phase holograms made,

using a plurality of transducers spread over a hologram area to convertthe elastic-wave energy into corresponding electrical signals suitableeither for directly recording a hologram or making a reproduciblerecording from which the hologram can be constructed. In particular, theinvention is directed to the very rapid recording of relative phases ofa large number of individual transducers.

2. Description of the Prior Art A hologram may be considered to be aform of interference or standing-wave pattern of the coherent energyredirected to the hologram area by an object or scene to be observed,combined with reference energy received more or less directly from thesource, such as via a plane reflector. In elastic-wave holography wheretransducers are used to produce equivalent electrical signals, anadditional option exists to combine the detected and the reference waveswhen they are in electrical form. This option is ordinarily notavailable in optical holography.

At any point of a hologram, the recorded intensity is a function of boththe phase-angle difference and the amplitudes of the interferingdetected and reference .waves. It is well known, however, that a quitesatisfactory hologram results if the wave amplitudes are neglected andonly the relative phase data are recorded, as variations in transparencyor gray scale, for example. When the waves being detected are inelectrical form and are steady-state waves, it is a simple matter tomeasure their relative phase by a conventional phase meter or analogousphase-responsive circuit. Such measurements, however, may require anumber of cycles and thus an appreciable interval of time for themetering device orcircuit to reach a final reading or indication.

In elastic-wave holography, this can be a problem in at least two ways:A very large number of detector points require a substantial length oftime for coverage of the detectors one point at a time; and there may beconditions where the phase pattern varies with time so that it isdesirable or necessary to record it at a specific or preciselypredetermined time. For example, where there may be noise interferencedue to travel of the elastic waves along two different paths from theirradiating source, which paths have different travel times, adverseeffects of the interference can sometimes be avoided by recording thehologram phase pattern when one or more of the interfering noise wavesis absent due to its different travel time.

Besides the saving in operating time and reduction of interference-waveeffects, there may be many other reasons why it is desirable ornecessary to record the relative phase of a large number of transducerscovering a hologram area in as short a time interval as possible. Itmay, accordingly, be considered to be a primary object of my inventionto provide a novel and improved method and apparatus for phase hologramrecording in elastic-wave holography, capable of recording the relativephases of a large number of transducers spread over the hologram area ina very reproducible form, creased efficiency.

utilizing the recording medium with inshot time interval, and whenrecording in 2 SUMMARY OF THE INVENTION waves the relative times whenthe output voltages of all the various holographic transducers passthrough zero in a given direction, say from negative to-positive.Preferably, this is done by subdividing the coherent wave period into anumber of phase intervals, each representative of a corresponding stepin a final hologram parameter such as gray-scale density, and finding inwhich interval the zero-axis crossing occurs for each of the manyrespective transducer voltage outputs. Thus, each transducer output isamplified to a high level and clipped or limited to produce essentiallya reversing-polarity squarewave. Differentiating the square-waveproduces a pulse at each reversal corresponding to the zero-axiscrossings, those of one polarity being chosen for use and the otheralternate ones disregarded. The chosen pulses are then shaped orstretched into brief unidirectional square pulses substantially equal inlength or duration to the phase interval. All of the transducer outputsare scanned in order, once for each phase interval, to detect the squarepulses indicating which ones, if any, of the transducers in the hologramarray have a relative phase within that interval, the times ofoccurrence during the scan representing transducer positions in thehologram area and thus the positions for corresponding gray-scaledensity points in the final hologram. Successive scans coversuccessively different phase-angle intervals until all possible relativephases and all transducer positions have been scanned and indicated.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings showing a preferredembodiment of the invention,

FIG. 1 shows a schematic cross section a marine environment and, inblock-diagram form, a typical preferred embodiment of the recordingsystem of the invention;

FIG. 2 presents graphs of the wave shapes and relationships involved inthe invention and occurring at various points in the circuit diagram ofFIG. 1;

FIGS. 3 and 4 represent two alternative forms for reproducibly recordingthe information produced in the system of FIG. 1;

FIG. 5 shown an alternative form of multiplexing means useful inconnection with the recording form of FIG. 4; and

FIG. 6 shows diagrammatically a reproducing system for records in theform of FIG. 4, for forming a visual display or hologram.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S) Referring now to the drawingsand particularly to FIG. I, this figure shows a typical preferredembodiment of the invention applied to sonic holography in a marineenvironment. An object 10 to be visualized is immersed in water 11 andrests on the marine bottom 12. Arranged at or near the water surface I3is a vibrator I5 and an array 17 of receivers 16, typically hydrophones,suitably located with respect to source 15 and object 20.Constant-frequency signals generated by a control oscillator 18 areamplified by a power amplifier l9 and drive the vibrator 15 to create inthe water 11 coherent elastic or sonic waves. From each point of bottom12 and of object 10, these coherent sonic waves are reflected and/orscattered, to be received by holographic receiver array 17 andtransmitted over a multiple-conductor cable 21 to a recording systemembodying the invention.

Elastic-wave holography with the apparatus so far described is fullyexplained in my U. S. Pat. Nos. 3,400,363; 3,450,225; and 3,461,420. Itshould also be understood that the explanation of the present inventionin terms of sonic holography in water is only by way of example, as itis applicable to virtually any form ofelastic-wave holography, such asacoustic or seismic. The invention comes into operation duringrecording,

when a reference signal from the oscillator 18 is transmitted to anamplifier 25, a limiter 26, a differentiator 27, a rectifier 28, and apulse shaper 29, in series, and placed in a sample-andhold 30 at thedesired time for making a phase-pattern measurement. Likewise, each ofthe n signals from the n transducers 16 of array 17 is similarlytransmitted to sample-and-hold 30 via a separate channel, as is typifiedby the amplifier 25a, the limiter 26a, the differentiator 27a, therectifier 28a, and the pulse shaper 29a, connected in series between theappropriate lead of cable 21 from one holographic detector a and theinput terminal of sample-and-hold30. Thence, a multiplexer unit 35 scansthe samples held in unit 30 and transmits them for reproduciblerecording by a recorder 36.

The action of sample-and-hold 30, multiplexer 35, and recorder 36 issynchronized with the coherent exciting signal of oscillator 18 bytransmitting the oscillator signal to a frequency multiplier 37 andthence to a clock-pulse generator 38, which produces controlimpulses atthe designated multiple L of the frequency of oscillator 18. Through aswitch 3.9 controlled by a trigger 40 actuated from the output pulse ofrectifier 28, clock pulses 38 are transmittedto control the operation ofsample-and hold 30, as well as of a multiplex scan and recorder controlunit 41 that coordinates by a mechanical connection 44 the scanning, bya multiplex contactor 42, of the output point 43 of sample-and-hold 30,with the recording by recorder 36 of the multiplexed signals.

The operation of this embodiment of the invention will now be explainedby reference'to both FIGS. 1 and 2. The various lines of the latterfigure designated (a), (b), and so on, are time functions of wave formspresent at various places in the system of FIG. 1, assuming time to runfrom left to right on FIG. 2 from a time-zero origin as shown at thebottom of the figure. Line (a) is thus a sine wave 46 representing theoutput of oscillator 18 going to amplifier 2S and limiter 36, thesquare-wave 49 of line (a) being the corresponding limiter output,assuming the wave form 46 to be amplified to very large amplitude in 25before truncation to the form of squarewave 49. Pulses 52 and 53 of line(c) appear at the output of differentiator 27 acting on square-wave 49,the rectifier 28 preventing transmission of the negative pulses 53 whiletransmitting only the positive pulses 52 to the pulse shaper 29. Therethe brief impulses 52 are converted to the square-wave pulses 57 on line(e) of precisely chosen length. This length is selected to be a definitefraction l/L of the total wave period T, the denominator of whichfraction is the factor of frequency multiplier 37. That is, on the basisthat one period T of wave 46 represents 360 of phase, the length ofsquare pulse 57 is K 360/L, there being a total integral number L ofsuch K intervals in the period T. Expressed as time in units of theperiod T, K" of phase is equivalent to M seconds where M T/L. lt is thefunction of this recording system to record the relative phase of theoutput of each detector 16 in units of K, there being a definite valueof gray-scale step associated with each unit value of K in the finaldisplay or hologram.

Thus, the output of each detector is subjected to the same operations asthe output of oscillator 18, line (b) showing the sinusoidal wave form47 and corresponding square-wave 50 that may be considered to representthe signal of that one of detectors l6 transmitted through the amplifier25a, limiter 26a, differentiator 27a, rectifier 28a, and pulse shaper29a to sample-and-hold 30. Line (d) corresponds to the output ofdifferentiator 27a, rectifier 28a cutting off negative pulses 55 andtransmitting only the positive pulses 54. Line (I) is the output ofpulse shaper 290, the square-wave pulse 58 representing approximately,though not exactly, a relative time delay between the waves 47 and 46 oftwo K units of phase. Square-wave pulses 59 and 60 on lines (3) and (It)may be regarded as representing the respective outputs of pulse shapers29b and 290 in two other detector channels. These signal channels areall either in phase with the reference signal or are out of phasetherewith by varying amounts, and it is the purpose of this invention todetermine these amounts in terms of the nearest integer multiple of thephase angle unit K.

Furthermore, this is to be done within one cycle of the frequency ofoscillator 18. The action is initiated by the pulse 52 of reference wave46 at the output of rectifier 28, which pulse actuates trigger mechanism40 to close the switch 39 during one cycle of the reference, thesucceeding pulse acting to reopen switch 39. Clock pulses from unit 38,accordingly, are transmitted to sample-and-hold 30 and to the controlunit 41 to cause recording of the phases by recorder 36 duringthedesired recording interval. The first clock pulse, which is synchronouswith pulses 52 and 57 of the reference wave 46, actuates sample-and-hold30, preferably at a time M/2 after one-half of a phase-angle intervalK", to place on the multiplexer contacts 43 voltage indications of thepresence of reference pulse 57 and of any other of the signalpulseswithin the phase-angle interval K preceding it. This first samplecorresponds, on the time scale of FIG. 2, to the vertical dashed linelabeled S,". One phase-angle interval K later at time 3M/2, a secondsampling S, is made of all channel outputs. ln

' the time interval M between the S and S samples, the control unit 41has caused the contactor 42 to scan all of the contacts 43 ofmultiplexer 35 and the pulse 57 plug any others encountered are storedby recorder 36. The second sampling S encounters and records the pulse60 on line (h) representing the time (phase) delay of the signal on thethird or c detector channel. The next sampling S finds pulse 58 for thefirst or a detector channel, while the sixth sampling S, finds the pulse59 for the second or b detector channel. It is not essential that Soccur exactly at time M/2 in the center of pulse 57 as that pulse willbe indicated by any scan falling within the pulse duration. lt isimportant, however, that the successive scans S -S be uniformly spaced Mseconds (K) apart.

The output of multiplexer 35 can be recorded in any of a great manydifferent ways, two of which are shown in FIGS. 3 and 4, the latterconstituting a preferred embodiment. Thus, as appears in FIG. 3, theresults of each successive sampling 8., S and so on, are recorded as asingle track 65 on a narrow magnetic tape 64, the direction of movementof the tape relative to the recording head being as indicated by thearrow 67. A magnetic mark, such as 66 corresponding to the referencepulse 57 in sample 8,, indicates which channels have substantially zerorelative delay compared to the reference and thus designate the firstgray-scale step. Sample block S shows the magnetic mark associated withpulse 60 of HG. 2, while sample 8;, shows the occurrence of pulse 58associated with channel l. The position of each magnetic mark 66 withinthe recorded sample interval block thus corresponds to a given detectorposition in array 17, while the particular sample interval between 1 andL designates the associated gray-scale step. Gaps (not shown) could, ifdesired, be inserted in the track 65 between successive blocks of sampledata to help differentiate the successive samples.

In the preferred arrangement shown in FIG. 4, a wide magnetic tape 69travels past an array of recording heads 70 extending in the transversetape direction, the direction of movement of the tape relative to array70 being as designated by arrow 71. As is indicated on the right side ofthe figure, this results in placing the successive sample intervals 8,,S and so on, along parallel lines extending transversely across tape 69rather than in sequential blocks as in FIG. 3. As shown in FIG. 5, byarranging for contactor 42 to connect each of the heads of array 70 insuccession at the same time that it'sweeps the contact points 43 ofmultiplexer 35, the recorded magnetization pattern of FIG. 4 isobtained. That is, positions across the tape transversely correspond todetector positions in array 17, while distance along the'length of thetape 69 corresponds to increasing relative phase angle.

lt may be noted that, while there are L phase-angle intervals of K inone cycle of the fundamental frequency 46, there are only about L/2gray-scale steps for the reason that the maximum relative phase occursfor a phase angle of l, and 360 of phase is the same as 0. That is, if 0or 360 corresponds to transparency or white on a variable-densityhologram presentation, then 180 corresponds to black or opacity.Relative phases of 90 and 270 have the same intermediate shade'of grayor partial transparency between the ends of the scale. lt should beapparent also that the illustrative magnetization pattern of H6. 4 isthat which actually occurs after the tape has passed underneathrecording head array, 70 moving in the direction of arrow 71.

One apparatus suitable for producing an optical hologram of the datarecorded on the tape 69 is shown in FIG. 6. The magnetic heads of array70 now function as reading heads for the recorded magnetic pulses on thetape 69 passing in the direction of arrow 71 underneath array 70. Thereading head of reference channel R is connected to a sawtooth voltagegenerator 74 which functions as the voltage supply to the intensityelectrode 75 of a cathode-ray tube 76, as determined by a beam on-offswitching unit 77. The remainder of the headset array 70 are connectedto a corresponding array of contacts 79 adapted to be scanned by acontactor 78 to transmit actuating pulses to the'switching unit 77 whena pulse is encountered during the contactor scan. A second contactor 80movablein-synchronism with contactor 78 scans along acorrespondingcontact array 81 which'taps off varying voltages from apotentiometer 82 energized by a battery 83. The

voltage of contactor 80' is transmitted to beam-positioning electrodes84 of tube 76, it being understoodthat a second potentiometer and set ofcontacts (not shown) can similarly supply voltage to the orthogonalbeam-deflection plates 85.

The resulting pattern of varying intensity over the face of tube 76'canbe recorded as an optical hologram on film by the camera86.

The action of the playback apparatus of FIG. 6 is as follows: as thetape 69 passes under the array of heads 70, the magnetic mark in track Rgenerates a voltage pulse which starts the operation of sawtoothgenerator 74. Simultaneously, contactors 78 and 80 sweep along contactarrays 79 and 81, to detect any other pulses present in the scan 8,. Anypulse or magnetic mark encountered triggers unit 77 to apply tointensity electrode 75 the then-existing voltage of sawtooth 74, thebeam being positioned at the location corresponding to the matchingdetector 16 by the then-existing voltages on electrodes 84 and 85. Astape 69 continues to move past bead array 70, the marks in scan S, aredetected at their corresponding channel positions, and the now greatervoltage of sawtooth generator 74 produces a correspondingly differentillumination of the face of cathode-ray tube 76 at the correspondingpositions. This process continues until the final scan 8,, passes underhead array 70, when the'hologram or at least that portion of itrepresented by the detector array 17 has been recorded on the film ofcamera 86. lt will be understood that the voltage output of sawtoothgenerator 74 must differ somewhat from the conventional sawtooth voltagein that, in order to produce the proper gray scale on the face ofcathoderay tube 76, the generator must reach its maximum voltage duringthe particular scan 5 when the phase is l80", and thereafter the voltagemust decrease linearly to its initial zero value for the relative phaseof 360 which is equivalent to 0 phase.

While the contactors 42, 78 and 80 have been illustrated as mechanicalswitches, this is only to simplify the explanation of the invention, asthese will ordinarily be semi-conductor devices well known in thecomputer and related arts as high speed switches.

While the reference signal in this embodiment has been shown as deriveddirectly from oscillator 18, it may alternatively be provided in otherways, such as from the received transducer signals as taught in my U. S.Pat. No. 3,450,225. Also, while the various circuit components of thesystem have been shown only in block-diagram form,all of these are wellknown in the art and are available as separate articles of commerce, sothat no further detailed description of them is deemed necessary.

While I have illustrated a system in which the source and the detectorsare stationary with respect to the object, this invention can be appliedequally well to systems in which either the one of the detector signalsas the reference since all that is required is the relative phasebetween each of the detectors.

Where the number of detector positions is' greater than the number ofdetector channels available, then the detectors must be recorded insequential groups, and a reference signal as shown in FlGS. l and 2 isrequired to tie together the phase measurements made in'successivegroups.

While I have illustrated the'utilization of the phase deter-. minationsin terms of spots in various positions in precisely determined values ofgray scale, to form an optical hologram for optical reconstruction, itwill be clear that the determined values of relative phase can equallywell be utilized in other ways, such as computer reconstruction.

lclaim:

1. ln a method of recording elastic-wave holographic data provided by aplurality of elastic'wave detector signal channels and a-referencesignal of frequency f, said recording to be of a parameter related tothe relative phases between said reference signal and the signal in eachof said detector channels, there being n channels and L independentequal-time spaced determinations in one cycle of said reference signal,the improvement comprising the steps of amplifying and clipping each ofsaid detector signals and said reference signal to provide correspondingsquarewave signals,

differentiating each of said square-wave signals to provide briefvoltage pulsesat each zero-axis crossing of each of said signals,

shaping each of said brief pulses of a given polarity into square-wavepulses of time duration M=l/fL,

at a time of about MI 2 following brief pulse of said reference signal,scanning each of said signal channels in sequence for the presence orabsence of one of said square-wave pulses,

recording on a record medium in the sequence of scanning said channels,an indication of the presence of square- V wave pulses when they occur,and

repeating said scanning and recording steps at each of a plurality ofsuccessive times M time units apart, until at least a total of L suchscans have been completed and recorded. 2. ln elastic-wave phaseholography wherein are recorded indications of the relative phases ofcoherent elastic waves of period T received by a plurality n oftransducers which convert said elastic waves to electrical signals, saidtransducers being spread over a hologram area, the improvementcomprising the steps of amplifying and clipping each of said ntransducer signals to produce corresponding square-wave signals,

differentiating said square-wave signals to produce brief positive andnegative pulses at the zero-axis crossings of said square-wave signals,

selecting said brief pulses of one polarity and shaping said selectedpulses into square-wave pulses of duration M TIL where L is an integerequal to the number of phaseangle intervals into which said period T issubdivided for determining said relative phases,

for at least L successive times spaced M seconds apart during at leastone period Tof said coherent waves, sampling the square-wave pulseoutputs of said u transducers to detect-the presence of said square-wavepulses, and

following each sampling, and in a sequence related to the position ofthe corresponding transducer in said hologram area, recording anindication of the occurrence of each square-wave pulse insaid sample,there being a range of relative phase angles and a correspondinghologram parameter associated with each sample.

3. ln elastic-wave phase holography, the improvement as in claim 2 inwhich said coherent elastic waves are generated in deriving a referencesignal from the output of said oscillator,

clipping said reference signal to produce a corresponding referencesquare-wave,

differentiating said reference square wave to produce brief referencepulses coincident with the zero-axis crossings of said reference signal,and

employing one of said brief reference pulses to initiate the first ofsaid L successive samplings of said u transducer square-wave pulseoutputs.

4. ln elastic-wave phase holography, the improvement as in claim 2 inwhich said coherent elastic waves are generated in response to theoutput of a constant-frequency control oscillator of period Tandincluding the further steps of deriving a reference signal from theoutput of said oscillator,

multiplying said reference signal by the integral factor L to produce aconstant-frequency sampling-control signal of period M TIL, andemploying said sampling-control signal to initiate each of said Lsuccessive samplings of said n transducer squarewave pulse outputs.

5. ln elastic-wave phase holography, the improvement as in claim 2 inwhich said step of recording comprises the step of reproduciblyrecording a quantity related to the occurrence of each square-wave pulsein said sample.

6. ln elastic-wave phase holography, the improvement as in claim 5including the additional steps of reading the reproducible recording ofsaid quantity, and

plotting on a record medium, in a position related to the position insaid hologram area of the corresponding transducer, a quantity relatedto the occurrence of each square-wave pulse in said sample.

7. In elastic-wave phase holography, the improvement as in claim 6 inwhich said quantity plotted in each position on said medium is an areaof gray-scale value related to the relative time of occurrence of thesquare-wave pulse in the coherentwave cycle.

8. ln elastic-wave phase holography, the improvement as in claim 5 inwhich said step of reproducibly recording comprises the steps ofrecording on a single track successive arrays of spots, each arraycomprising spots corresponding to the presence of square-wave pulses onsequentially sampled channels, and each successive array correspondingto one of said L samplings.

9. ln elastic-wave phase holography, the improvement as in claim 5 inwhich said step of reproducibly recording comprises the steps ofrecording on each of multiple tracks, one corresponding to each of thetransducer signal channels, at a time corresponding to one of the Lsuccessive samplings, a mark corresponding to the presence of asquare-wave pulse in said channel.

10. In elastic-wave phase holography employing a holographic detectingarray of n electrical transducers in an elastic-wave transmittingmedium, each of said transducers acting to convert coherent elastic-waveenergy of period T incident thereon into n corresponding electricalsignals, the improved multiple-channel system for recording indicationsof the relative phases of said signals comprising n signal-channelmeansto amplify and clip each of said transducer electrical signals toproduce corresponding square-wave signals,

means to differentiate and rectify said square-wave signals to produce abrief pulse at each alternate zero-axis crossing of each of saidsquare-wave signals of a given polarity,

means to shape said brief pulses into square-wave pulses of duration MT/L seconds where L is an integer equal to the number of phase-angleintervals into which said period T is subdivided for determining saidrelative ases m ans to sample, for at least L successive times spaced intime by M seconds, all of the outputs of said signal channels to detectthe presence of said square-wave pulses, and

means to record, as a function of detector position in said array andcorresponding channel position in the order of sampling, an indicationfor each of said n channels as to which of said 1 samples contained thecorresponding

1. In a method of recording elastic-wave holographic data provided by aplurality of elastic-wave detector signal channels and a referencesignal of frequency f, said recording to be of a parameter related tothe relative phases between said reference signal and the signal in eachof said detector channels, there being n channels and L independentequal-time spaced determinations in one cycle of said reference signal,the improvement comprising the steps of amplifying and clipping each ofsaid detector signals and said reference signal to provide correspondingsquare-wave signals, differentiating each of said square-wave signals toprovide brief voltage pulses at each zero-axis crossing of each of saidsignals, shaping each of said brief pulses of a given polarity intosquare-wave pulses of time duration M 1/fL, at a time of about M/ 2following brief pulse of said reference signal, scanning each of saidsignal channels in sequence for the presence or absence of one of saidsquare-wave pulses, recording on a record medium in the sequence ofscanning said channels, an indication of the presence of square-wavepulses when they occur, and repeating said scanning and recording stepsat each of a plurality of successive times M time units apart, until atleast a total of L such scans have been completed and recorded.
 2. Inelastic-wave phase holography wherein are recorded indications of therelative phases of coherent elastic waves of period T received by aplurality n of transducers which convert said elastic waves toelectrical signals, said transducers being spread over a hologram area,the improvement comprising the steps of amplifying and clipping each ofsaid n transducer signals to produce corresponding square-wave signals,differentiating said square-wave signals to produce brief positive andnegative pulses at the zero-axis crossings of said square-wave signals,selecting said brief pulses of one polarity and shaping said selectedpulses into square-wave pulses of duration M T/L where L is an integerequal to the number of phase-angle intervals into which said period T issubdivided for determining said relative phases, for at least Lsuccessive times spaced M seconds apart during at least one period T ofsaid coherent waves, sampling the square-wave pulse outputs of said ntransducers to detect the presence of said square-wave pulses, andfollowing each sampling, and in a sequence related to the position ofthe corresponding transducer in said hologram area, recording anindication of the occurrence of each square-wave pulse in said sample,there being a range of relative phase angles and a correspondinghologram parameter associated with each sample.
 3. In elastic-wave phaseholography, the improvement as in claim 2 in which said coherent elasticwaves are generated in response to the output of a constant-frequencycontrol oscillator and including the further steps of deriving areference signal from the output of said oscillator, clipping saidreference signal to produce a corresponding reference square-wave,diffErentiating said reference square wave to produce brief referencepulses coincident with the zero-axis crossings of said reference signal,and employing one of said brief reference pulses to initiate the firstof said L successive samplings of said n transducer square-wave pulseoutputs.
 4. In elastic-wave phase holography, the improvement as inclaim 2 in which said coherent elastic waves are generated in responseto the output of a constant-frequency control oscillator of period T andincluding the further steps of deriving a reference signal from theoutput of said oscillator, multiplying said reference signal by theintegral factor L to produce a constant-frequency sampling-controlsignal of period M T/L, and employing said sampling-control signal toinitiate each of said L successive samplings of said n transducersquare-wave pulse outputs.
 5. In elastic-wave phase holography, theimprovement as in claim 2 in which said step of recording comprises thestep of reproducibly recording a quantity related to the occurrence ofeach square-wave pulse in said sample.
 6. In elastic-wave phaseholography, the improvement as in claim 5 including the additional stepsof reading the reproducible recording of said quantity, and plotting ona record medium, in a position related to the position in said hologramarea of the corresponding transducer, a quantity related to theoccurrence of each square-wave pulse in said sample.
 7. In elastic-wavephase holography, the improvement as in claim 6 in which said quantityplotted in each position on said medium is an area of gray-scale valuerelated to the relative time of occurrence of the square-wave pulse inthe coherent-wave cycle.
 8. In elastic-wave phase holography, theimprovement as in claim 5 in which said step of reproducibly recordingcomprises the steps of recording on a single track successive arrays ofspots, each array comprising spots corresponding to the presence ofsquare-wave pulses on sequentially sampled channels, and each successivearray corresponding to one of said L samplings.
 9. In elastic-wave phaseholography, the improvement as in claim 5 in which said step ofreproducibly recording comprises the steps of recording on each ofmultiple tracks, one corresponding to each of the transducer signalchannels, at a time corresponding to one of the L successive samplings,a mark corresponding to the presence of a square-wave pulse in saidchannel.
 10. In elastic-wave phase holography employing a holographicdetecting array of n electrical transducers in an elastic-wavetransmitting medium, each of said transducers acting to convert coherentelastic-wave energy of period T incident thereon into n correspondingelectrical signals, the improved multiple-channel system for recordingindications of the relative phases of said signals comprising nsignal-channel means to amplify and clip each of said transducerelectrical signals to produce corresponding square-wave signals, meansto differentiate and rectify said square-wave signals to produce a briefpulse at each alternate zero-axis crossing of each of said square-wavesignals of a given polarity, means to shape said brief pulses intosquare-wave pulses of duration M T/L seconds where L is an integer equalto the number of phase-angle intervals into which said period T issubdivided for determining said relative phases, means to sample, for atleast L successive times spaced in time by M seconds, all of the outputsof said signal channels to detect the presence of said square-wavepulses, and means to record, as a function of detector position in saidarray and corresponding channel position in the order of sampling, anindication for each of said n channels as to which of said l samplescontained the corresponding square-wave pulse.
 11. An improved recordingsystem for elastic-wave phase holography as in claim 10 including alsomeans to display, as gray-valued spots in positions related to thepositions of the corresponding transducers in said array, saidindications of square-wave pulse occurrence, the said gray value beingrelated to the sequential number of sample in said succession of Lsamples.